1. Field of the Invention
The present invention relates to a switchable type optical device with variable magnification suitable for use with a photographic camera or a video camera, and more particularly to a small-size switchable type optical device with variable magnification which comprises three photographic units each having a different focal length, the three photographic units including a master lens unit as a common unit and two auxiliary lens units disposed on the side of an image plane of the master lens unit in such a manner that they can be freely placed on and off the optical axis of the master lens unit, thereby easily achieving a relatively high ratio of variable magnification and a high optical performance.
2. Description of the Related Art
Various proposals have heretofore been made with respect to a switchable type optical device with variable magnification employing a so-called conversion lens, namely, an auxiliary lens unit which is attached to the object or image-plane side of the master lens unit so as to be capable of varying the total focal length of the optical device while maintaining the total focal plane of the same at a fixed position. However, a so-called front conversion method in which a conversion lens is attached to the object side of the master lens unit has a particular tendency to increase the lens diameter of the conversion lens, and thus difficulty is experienced in reducing the overall size of the optical device.
On the other hand, a so-called rear conversion method in which a conversion lens is attached to the image-plane side of the master lens unit provides the merit of reducing the lens diameter as compared with the aforesaid front conversion method but involves difficulties in terms of correcting various aberrations. As a result, the number of lens elements employed is increased. As described in U.S. Pat. No. 4,340,279, this in turn requires an increase in the overall length of the lens system. To overcome this problem, U.S. Pat. No. 4,422,734 proposes a lens arrangement consisting of a reduced number of lens elements, but difficulty is encountered in increasing the ratio of variable magnification with this prior art.
Japanese Laid-open Patent Application No. SHO 59-29214 proposes a wide-angle conversion lens employing a rear conversion method in which a lens unit having a positive refractive power is disposed at the rear of the master lens unit so as to shorten the total focal length of the optical device.
However, the wide-angle conversion lens proposed in the above patent application involves the disadvantage in that, when the conversion lens is attached to the master lens unit, the conversion ratio obtained is small, i.e. 2 or less, while the lens system as a whole is increased in length and size.
Japanese Patent Publication No. SHO 58-38778 proposes a small-size switchable type optical device with variable magnification in which two lens units such as a standard lens and a telephoto lens are disposed in side-by-side relation facing an object to be photographed, either of these lenses being selectively used by means of a movable reflecting mirror incorporated in the camera body. However, this optical device with variable magnification involves the disadvantage that, since the focal lengths of these lens units differ, the individual lens units must be extended by different distances with respect to an object disposed at a distance from the same camera.
In cases where the aforesaid optical device is utilized in, for example, an autofocus camera, the respective lens units are arranged to be capable of extending by respectively different distances in standard-lens and telephoto-lens conditions. Thus, there is a tendency for the internal mechanism of the camer per se to become complicated.
Japanese Laid-open Patent Application No. SHO 53-33624 gives a diagrammatic illustration of a photographic device in which a positive lens is placed on and off the optical axis on the image-plane side of the master lens unit while a positive attachment lens unit is detachably mounted on the object side of the master lens unit. However, this proposal has the disadvantage that the use of a structure in which a fixed ring of the attachment lens unit is screwed into one end of a lens barrel precludes rapid switchover.